In recent years, a mobile communication system called a W-CDMA (Wideband-Code Division Multiple Access) system or a UMTS (Universal Mobile Telecommunications System) based on a code division multiple access (Code Division Multiple Access: CDMA) technique has been proposed and specified by the 3GPP (3rd Generation Partnership Projects).
FIG. 8 shows a configuration example of a W-CDMA system. The W-CDMA system includes a mobile station UE (User Equipment), radio base stations NodeB, a radio control station RNC (Radio Network Controller), and an exchange MSC/SGSN (Mobile Switching Center/Serving GPRS Support Node), thereby constituting a cellular mobile communication system.
An area called a cell (Cell) that defines a control unit in the cellular system is formed by a radio wave sent by each radio base station NodeB.
Here, in the W-CDMA system, multiple radio control stations RNC and multiple exchanges MSC/SGSN usually exist and are connected to one another. However, the single radio control station RNC and the single exchange MSC/SGSN are only shown in the example of FIG. 8 for the purpose of simplification.
In addition, the W-CDMA system is usually equipped with a device called a home memory to store subscriber information therein, apart from the exchange MSC/SGSN. However, a function of the home memory is assumed to be included in the exchange MSC/SGSN in the example of FIG. 8 similarly for the purpose of simplification.
Moreover, in the W-CDMA system, a mobile station UE which is turned on but not performing communication, i.e., which is in a standby state of waiting for a call originating action through a terminal operation by a user or for reception of an incoming call, measures an electric field in a pilot channel sent by the radio base station NodeB and autonomously selects an optimal pilot channel. Here, a selection algorithm is not related to the present invention and description thereof is omitted.
Then, the mobile station UE which selects the optimal pilot channel, namely the cell, reads notification information regarding the cell and turns into standby.
The mobile station UE executes location registration processing for getting ready to standby. FIG. 9 shows a sequence of the location registration processing and call processing according to the conventional technology.
As shown in FIG. 9, in step S2001, the radio control station RNC refers to a location number table (see FIG. 10(a)) and extracts location information “0x0001” by using a cell identifier “#1” as key information, and transmits notification information (SYSTEM INFORMATION) containing the extracted location number “0x0001” to a cell #1 identified by the cell identifier “#1”.
In step S2002, the mobile station UE located in the cell #1 receives the notification information containing the location number “0x0001” and determines whether or not the location number “0x0001” matches a location number stored in the mobile station UE.
When the numbers match each other, the mobile station UE continues standby in the cell #1 in step S2003.
When the numbers do not match each other, the mobile station UE establishes a radio link with the radio control station RNC in step S2004 and transmits a location registration request (ROUTING AREA UPDATE REQUEST) to the radio control station RNC in step S2005.
Here, the radio control station RNC refers to the location number table (see FIG. 10(a)) and extracts the location number “0x0001” by using, as the key information, the cell identifier “#1” for identifying the cell #1 being an originator of the received location registration request, and notifies the exchange MSC/SGSN of the extracted location number “0x0001” and a user identifier “UE#1” for identifying the mobile station UE.
The exchange MSC/SGSN executes authentication processing for the mobile station UE in step S2006. When the authentication processing is successful, the exchange MSC/SGSN transmits a location registration acceptance signal (ROUTING AREA UPDATE ACCEPT) containing the location number “0x0001” to the mobile station UE in step S2007.
The mobile station UE stores the location number “0x0001” contained in a received location registration completion response in step S2008, and transmits a location registration completion signal (ROUTING AREA UPDATE COMPLETE) to the exchange MSC/SGSN in step S2009.
In step S2010, the exchange MSC/SGSN registers the location number “0x0001” and the user identifier “UE#1” identifying the mobile station UE, in association with each other.
In step S2011, the radio link between the mobile station UE and the radio control station RNC is released.
Thereafter, in step S2012, upon receipt of an incoming call signal addressed to the mobile station UE, the exchange MSC/SGSN refers to a location registration table (see FIG. 11), extracts the location number “0x0001” by using the user identifier “UE#1” contained in the incoming call signal as key information, refers to a RNC identification table, and extracts a RNC identifier “RNC#1” by using the extracted location number “0x0001” as key information.
Then, the exchange MSC/SGSN transmits a paging request (PAGING REQUEST) containing the extracted location number “0x0001” to the radio control station RNC identified by the extracted RNC identifier “RNC#1”.
In step S2013, the radio control station RNC refers to a reverse lookup location number table (see FIG. 10(b)) and extracts the cell identifier “#1” by using the location number “0x0001” contained in the received paging request as key information.
Then, the radio control station RNC transmits a paging signal (PAGING) to the cell #1 which is identified by the cell identifier “#1”.
In accordance with the sequence described above, the radio control station RNC can avoid transmission of a paging signal to cells #2 to #4, identified by cell identifiers #2 to #4, where the mobile station UE is not located.
Specifically, the location registration processing aims to enable restriction of the cell to which the paging signal is transmitted at the time of call processing.
An area formed by a group of cells set with the same location number is called a “location registration area”. A large amount of the call processing occurs when the location registration area is too large. On the other hand, a large amount of the location registration processing occurs when the location registration area is too small. Accordingly, the location registration area is usually optimized by considering a balance of a trade-off relationship between an increase in traffic due to the call processing and an increase in load due to the location registration processing.
Meanwhile, as another type of the radio base station, very small radio base stations (hereinafter a “femto radio base station”) mainly to cover an indoor space are increasingly introduced in addition to a public radio base station covering an outdoor space.
In addition, there has been studied a limited user service which forms a private area (formed of single or multiple cells) dedicated for a specific user to offer the specific user a special billing system in the area and additional functions available only in the area. Here, to form the private area, the femto radio base stations are installed in standard homes, small offices or the like, then are adjusted to minimize their coverage area and are connected to the radio control station RNC by using an inexpensive home IP line.
This limited user service, however, is subjected to a risk that the service performance for the specific user in the cells forming the area may be unintentionally degraded due to congestion caused by mobile stations of an unspecified number of users being on standby or performing communication in the cells. To avoid this risk in offering the limiter user service, one of conceivable ways is to assign different location numbers to all the cells to impose each mobile station to always perform the location registration processing when making a cell selection, and thereby to determine whether or not to allow the mobile station to turn into standby along with the location registration processing.
However, in this case, there is a problem of needing an enormous amount of the location numbers as the key information in the reverse lookup location number table. This is because all the location numbers “0x0001” to “0x0004” corresponding to cell identifiers #1 to #4 are different from one another as shown in FIG. 10, and because numerous femto radio base stations with small capacities usually exist.